Reef-island topography and the vulnerability of atolls to sea-level rise

Low-lying reef islands on the rim of atolls are perceived as particularly vulnerable to the impacts of sea-level rise. Three effects are inferred: erosion of the shoreline, inundation of low-lying areas, and saline intrusion into the freshwater lens. Regional reconstruction of sea-level trends, supplementing the short observational instrumental record, indicates that monthly mean sea level is rising in the eastern Indian and western Pacific Oceans. This paper reviews the morphology and substrate characteristics of reef islands on Indo-Pacific atolls, and summarises their topography. On most atolls across this region, there is an oceanward ridge built by waves to a height of around 3 m above MSL; in a few cases these are topped by wind-blown dunes. The prominence of these ridges, together with radiocarbon dating and multi-temporal studies of shoreline position, indicate net accretion rather than long-term erosion on most of these oceanward shores. Less prominent lagoonward ridges occur, but their morphology and continuity are atoll-specific, being a function of the processes operating in each lagoon. Low-lying central areas are a feature of many islands, often locally excavated for production of taro. These lower-lying areas are already subject to inundation, which seems certain to increase as the sea rises. Tropical storms play an important role in the geomorphology of reef islands in those regions where they are experienced. Topographical differences, as well as features such as emergence of the reef flat and the stability of the substrate, mean that islands differ in terms of their susceptibility to sea-level rise. Further assessment of variations in shoreline vulnerability based on topography and substrate could form the basis for enhancing the natural resilience of these islands.     

Postglacial relative sea-level change and stratigraphy of raised coastal basins on Kola Peninsula, northwest Russia

A relative sea-level curve for the Holocene is constructed for Polyarny on the Kola Peninsula, northwest Russia. The curve is based on 18 radiocarbon dates of isolation contacts, identified from lithological and diatomological criteria, in nine lake basins situated between 12 and 57 m a.s.l. Most of the lakes show a conformable, regressive I–II–III (marine–transitional–freshwater) facies succession, indicating a postglacial history comprising an early (10,000–9000 radiocarbon years BP) phase of rapid, glacio-isostatically induced emergence (5 cm year⁻¹) and a later phase (after 7000 years BP,) having a moderate rate of emergence (<0.5 cm year⁻¹). Three lakes together record a phase of very low rate of emergence or slight sea-level rise at a level of 27 m a.s.l., between 8500 and 7000 years BP, which correlates with the regional Tapes transgression. Pollen stratigraphy in the highest lake shows that the area was deglaciated before the Younger Dryas and that previously reconstructed Younger Dryas glacier margins along the north Kola coast lie too far north.     

Saturn's Titan: Surface change, ammonia, and implications for atmospheric and tectonic activity

Titan is known to have a young surface. Here we present evidence from the Cassini Visual and Infrared Mapping Spectrometer that it is currently geologically active. We report that changes in the near-infrared reflectance of a 73,000 km² area on Titan (latitude 26° S, longitude 78° W) occurred between July 2004 and March of 2006. The reflectance of the area increased by a factor of two between July 2004 and March-April 2005; it then returned to the July 2004 level by November 2005. By late December 2005 the reflectance had surged upward again, establishing a new maximum. Thereafter, it trended downward for the next three months. Detailed spectrophotometric analyses suggest these changes happen at or very near the surface. The spectral differences between the region and its surroundings rule out changes in the distribution of the ices of reasonably expected materials such as H₂O, CO₂, and CH₄ as possible causes. Remarkably, the change is spectrally consistent with the deposition and removal of NH₃ frost over a water ice substrate. NH₃ has been proposed as a constituent of Titan's interior and has never been reported on the surface. The detection of NH₃ frost on the surface might possibly be explained by episodic effusive events occur which bring juvenile ammonia from the interior to the surface. If so, its decomposition would feed nitrogen to the atmosphere now and in the future. The lateral extent of the region exceeds that of active areas on the Earth (Hawaii) or Io (Loki).     

Four climate regimes on a land planet with wet surface: Effects of obliquity change and implications for ancient Mars

Series of numerical experiments are performed using a general circulation model to gain insights on the hydrologic cycle on ancient Mars. Since the state of the ancient Mars atmosphere is not well constrained, we did not try to simulate an ancient Mars climate under warm and wet condition. In stead, we used an idealized model and tried to extract general features of the hydrologic cycle by modeling an ideal land planet that has no ocean on its surface. Four different climate regimes, “warm-upright,” “warm-oblique,” “frozen-upright,” and “frozen-oblique” regimes, are recognized depending on the inclination of the spin axis (obliquity) and average surface temperature. The period of active hydrologic cycle suggested from the geomorphology on Mars seems to be consistent with that at the “warm-oblique” regime, which appears at warm (above-freezing) environment with high-obliquity (higher than about 30°) condition.     

Antarctic dry valleys: Microclimate zonation, variable geomorphic processes, and implications for assessing climate change on Mars

The Antarctic Dry Valleys (ADV) are generally classified as a hyper-arid, cold-polar desert. The region has long been considered an important terrestrial analog for Mars because of its generally cold and dry climate and because it contains a suite of landforms at macro-, meso-, and microscales that closely resemble those occurring on the martian surface. The extreme hyperaridity of both Mars and the ADV has focused attention on the importance of salts and brines on soil development, phase transitions from liquid water to water ice, and ultimately, on process geomorphology and landscape evolution at a range of scales on both planets. The ADV can be subdivided into three microclimate zones: a coastal thaw zone, an inland mixed zone, and a stable upland zone; zones are defined on the basis of summertime measurements of atmospheric temperature, soil moisture, and relative humidity. Subtle variations in these climate parameters result in considerable differences in the distribution and morphology of: (1) macroscale features (e.g., slopes and gullies); (2) mesoscale features (e.g., polygons, including ice-wedge, sand-wedge, and sublimation-type polygons, as well as viscous-flow features, including solifluction lobes, gelifluction lobes, and debris-covered glaciers); and (3) microscale features (e.g., rock-weathering processes/features, including salt weathering, wind erosion, and surface pitting). Equilibrium landforms are those features that formed in balance with environmental conditions within fixed microclimate zones. Some equilibrium landforms, such as sublimation polygons, indicate the presence of extensive near-surface ice; identification of similar landforms on Mars may also provide a basis for detecting the location of shallow ice. Landforms that today appear in disequilibrium with local microclimate conditions in the ADV signify past and/or ongoing shifts in climate zonation; understanding these shifts is assisting in the documentation of the climate record for the ADV. A similar type of landform analysis can be applied to the surface of Mars where analogous microclimates and equilibrium landforms occur (1) in a variety of local environments, (2) in different latitudinal bands, and (3) in units of different ages. Documenting the nature and evolution of the ADV microclimate zones and their associated geomorphic processes is helping to provide a quantitative framework for assessing the evolution of climate on Mars.     

The effect of human activity on radiative forcing of climate change: a review of recent developments

Human activity has perturbed the Earth's energy balance by altering the properties of the atmosphere and the surface. This perturbation is of a size that would be expected to lead to significant changes in climate. In recent years, an increasing number of possible human-related climate change mechanisms have begun to be quantified. This paper reviews developments in radiative forcing that have occurred since the second assessment report of the Intergovernmental Panel on Climate Change (IPCC), and proposes modifications to the values of global-mean radiative forcings since pre-industrial times given by IPCC. The forcing mechanisms which are considered here include those due to changes in concentrations of well-mixed greenhouse gases, tropospheric and stratospheric ozone, aerosols composed of sulphate, soot, organics and mineral dust (including their direct and indirect effects), and surface albedo. For many of these mechanisms, the size, spatial pattern and, for some, even the sign of their effect remain uncertain. Studies which have attributed observed climate change to human activity have considered only a subset of these mechanisms; their conclusions may not prove to be robust when a broader set is included.     

Gravity in one dimension: Stability of a three particle system

The autonomous dynamical system consisting of parallel planes of constant mass density moving under their mutual gravitational attraction is of interest for testing diverse astrophysical models of gravitational relaxation. The simplest non-integrable system consists of three sheets. In this paper the dynamics and stability of this one-dimensional solar system is systematically investigated. A linear transformation of the coordinates reduces the problem to that of a falling body constrained by oblique boundaries. By constructing a Poincare surface of section, it is found that regions of stability and chaos coexist. This behavior is predicted by the KAM theorem for systems having smoother trajectories in phase space, but does not apply here because of the discontinuity in acceleration experienced by each sheet during an encounter. The results of numerical experiments indicate that chaotic regions may be associated with trajectories which contain nearly triple collisions of the three particles.     

Energetics of magnetized black holes: Effective ergospheres and superradiance

The model of a black hole rotating in an external magnetic field is considered. The mechanisms of the energy extraction from the black hole in terms of the effective ergospheres for charged particles are described. It is shown that the external magnetic field essentially influences the boundaries of the effective ergospheres and the superradiance threshold.     

Meteoroid streams at Mars: possibilities and implications

In order to assess the possibility of meteoroid streams detectable from the surface of Mars as meteor showers we have derived minimum distances and associated velocities for a large sample of small body orbits relative to the orbits of Mars and the Earth. The population ratio for objects approaching to within 0.2 AU of these two planets is found to be approximately 2:1. The smaller relative velocities in the case of Mars appears to be the main impediment to the detection of meteors in the upper atmosphere of that planet. We identify five bodies, including the unusual object (5335) Damocles and periodic comet 1P/Halley, with relative orbital parameters most suitable to produce prominent meteor showers. We identify specific epochs at which showers related to these bodies are expected to occur. An overview of possible detection methods taking into account the unique characteristics of the Martian environment is presented. We pay particular attention on the effects of such streams on the dust rings believed to be present around Mars.     

Lunar mascons: another model and its implications

A mascon model is proposed in which the mass excess of the mare basalts in the circular maria is supported isostatically by mass deficits at depth. The model predicts the observed positive gravity anomalies surrounded by negative ring anomalies and explains the absence of gravity anomalies over the irregular maria. The model implies that mare basalts were derived by partial melting of a source region at depth due to pressure relief resulting from the excavation of the circular mare basins, and that the crystalline residuum in the source region is of lower density than the original source rock. The trace element enrichment and near cotectic character of Apollo 11 and 12 lavas reported by some investigators may be caused by extensive magma fractionation enroute from an origin in the circular maria to the final, distant emplacement sites.     

Effective index of calcite and quartz

The thickness of calcite or quartz required for an element of a given half width (FWHM) depends not only on the wavelength and index difference of the birefringent material, but also the first derivative of the index difference with wavelength. For calcite, the effective index difference can differ by as much as 50% from published values.     

Morphology of Lonar Crater,India: Comparisons and implications

Lonar Crater is a young meteorite impact crater emplaced in Deccan basalt. Data from 5 drillholes, a gravity network, and field mapping are used to reconstruct its original dimensions, delineate the nature of the pre-impact target rocks, and interpret the emplacement mode of the ejecta. Our estimates of the pre-erosion dimensions are: average diameter of 1710 m; average rim height of 40 m (30–35 m of rim rock uplift, 5–10 m of ejected debris); depth of 230–245 m (from rim crest to crater floor). The crater's circularity index is 0.9 and is unlikely to have been lower in the past. There are minor irregularities in the original crater floor (present sediment-breccia boundary) possibly due to incipient rebound effects. A continuous ejecta blanket extends an average of 1410 m beyond the pre-erosion rim crest.In general, fresh terrestrial craters, less than 10 km in diameter, have smaller depth/diameter and larger rim height/diameter ratios than their lunar counterparts. Both ratios are intermediate for Mercurian craters, suggesting that crater shape is gravity dependent, all else being equal. Lonar demonstrates that all else is not always equal. Its depth/diameter ratio is normal but, because of less rim rock uplift, its rim height/diameter ratio is much smaller than both fresh terrestrial and lunar impact craters. The target rock column at Lonar consists of one or more layers of weathered, soft basalt capped by fresh, dense flows. Plastic deformation and/or compaction of this lower, incompetent material probably absorbed much of the energy normally available in the cratering process for rim rock uplift.A variety of features within the ejecta blanket and the immediately underlying substrate, plus the broad extent of the blanket boundaries, suggest that a fluidized debris surge was the dominant mechanism of ejecta transportation and deposition at Lonar. In these aspects, Lonar should be a good analog for the fluidized craters of Mars.     

PSCz superclusters: detection, shapes and cosmological implications

We study the possibility of correctly identifying, from the smooth galaxy density field of the PSC z flux-limited catalogue, high-density regions (superclusters) and recovering their true shapes in the presence of a bias introduced by the coupling between the selection function and the constant radius smoothing. We quantify such systematic biases in the smoothed PSC z density field and after applying the necessary corrections we study supercluster multiplicity and morphologies using a differential geometry definition of shape. Our results strongly suggest that filamentary morphology is the dominant feature of PSC z superclusters. Finally, we compare our results with those expected in three different cosmological models and find that the Λ cold dark matter (CDM) model (Ω_Λ=1−Ω_m=0.7) performs better than Ω_m=1 CDM models.     

Martian canyons and African rifts: Structural comparisons and implications

The resistant parts of the canyon walls of the Martian rift complex Valles Marineris have been used to infer an earlier, less eroded reconstruction of the major troughs. The individual canyons were then compared with individual rifts of East Africa. When measured in units of planetary radius, Martian canyons show a distribution of lengths nearly identical to those in Africa, both for individual rifts and for compound rift systems. A common mechanism which scales with planetary radius is suggested. Martian canyons are significantly wider than African rifts. This is consistent with the long-standing idea that rift width is related to crustal thickness: most evidence favors a crust on Mars at least 50% thicker than that of Africa. The overall pattern of the rift systems of Africa and Mars are quite different in that the African systems are composed of numerous small faults with highly variable trend. On Mars the trends are less variable; individual scarps are straighter for longer than on Earth. This is probably due to the difference in tectonic histories of the two planets: the complex history of the Earth and the resulting complicated basement structures influence the development of new rifts. The basement and lithosphere of Mars are inferred to be simple, reflecting a relatively inactive tectonic history prior to the formation of the canyonlands.     

Source structure of gradual rise and fall bursts at 17 GHz

From 200 GRF (gradual rise and fall) bursts which have been recorded with the 17 GHz interferometer at Nobeyama, we deduce the following characteristics of GRF bursts: (1) Sources of GRF bursts are broader, less circularly polarized than those of impulsive bursts. (2) The sources are potentially of bipolar structure and have the peak brightness near the position at which the sense of circular polarization changes. (3) The association of GRF bursts with type III bursts, which are indicative of nonthermal electron acceleration, is significantly poorer than that of impulsive bursts.It is suggested that the sources of GRF bursts or generally of thermal bursts lie relatively high in the solar atmosphere possibly near the top of magnetic loops or arches which divide two regions of opposite magnetic polarity.     

Near-infrared spectra of the Galilean satellites: Observations and compositional implications

We have obtained reflectivity spectra of the trailing and leading sides of all four Galilean satellites with circular variable filter wheel spectrometers operating in the 0.7- to 5.5-μm spectral interval. These observations were obtained at an altitude of 41,000 ft from the Kuiper Airborne Observatory. Features seen in these data include a 2.9-μm band present in the spectra of both sides of Callisto; the well-known 1.5-μm and 2.0-μm combination bands and the previously more poorly defined 3.1-μm fundamental of water ice observed in the spectra of both sides of Europa and Ganymede; and features centered at 1.35 ± 0.1, 2.55 ± 0.1, and 4.05 ± 0.05 μm noted in the spectra of both sides of Io. In an effort to interpret these data, we have compared them with laboratory spectra as well as synthetic spectra constructed with a simple multiple-scattering theory. We attribute the 2.9-μm feature of Callisto's spectra primarily to bound water, with the product of fractional abundance of bound water and mean grain radius in micrometers equaling approximately 3.5 × 10^?1 for both sides of the satellite. The fractional amounts of water ice cover on the trailing side of Ganymede, its leading side, and the leading side of Europa were found to be 50 ± 15, 65 ± 15, and 85% or greater, respectively. The bare ground areas on Ganymede have reflectivity properties in the 0.7- to 2.5-μm spectral region comparable to those of Callisto's surface and also have significant quantities of bound water, as does Callisto. Interpretation of the spectrum for the trailing side of Europa is complicated by magnetospheric particle bombardment which causes a perceptible broadening of strong bands, but the ice cover on this side is probably comparable to that on the leading side. These irradiation effects may be responsible for much of the difference in the visual geometric albedos of the two sides of Europa. Minor, but significant, amounts of ferrous-bearing material (either ferrous salts or alkali feldspars but not olivines or pyroxenes) account for the 1.35-μm feature of Io. The two longer wavelength bands are most likely attributable to nitrate salts. Ferrous salts and nitrates can jointly also account for much of the spectral variation in Io's visible reflectivity, thereby eliminating the need to postulate large quantities of sulfur. The absence of noticeable features near 3-μm wavelength in Io's spectra leads to upper bounds of 10% on the fractional cover of water and ammonia ice and 10^?3 on the relative abundance of bound water and hydroxylated material on Io. The two sides of Io have similar compositions. We suggest that the systematic increase in fractional water ice cover from Callisto to Ganymede to Europa is bought about by variations in efficiencies of recoating the satellite's surface by interior water brought to the surface, and by the deposition of extrinsic dust. The most important component of the latter is debris, derived from the outer irregular satellites of Jupiter, which impacts the Galilean satellites at relatively low velocities. Europa has the largest water ice cover because its crust is thinnest and thus the frequency of water recoating is the greatest, and because it is farthest from the sources of low-velocity dust. While models which depict Io's surface as consisting primarily of very fine-grained ice are no longer viable, we are unable to definitively distinguish between the salt assemblage and alkali feldspar models. The salt model can better account for Io's reflectivity spectrum from 0.3 to 5 μm, but the absence of appreciable quantities of bound water and hydroxylated material may not be readily understood within the context of that model.